Because solid polymer electrolyte membrane type fuel cells are of a construction such that a desired output is obtained by a number of individual fuel cell cells being stacked together, as separators for dividing the individual fuel cells, metal materials having superior strength with respect to pressurization on stacking and compactness after stacking compared to resin materials are being seen as effective. In particular, because it forms a passive film having a high resistance to corrosion in acidic atmospheres around the electrode parts of cells, the use of stainless steel members is being studied.
As passivation treatment methods for forming this kind of passive film on a stainless steel member, those in which an acidic solution is used as a process liquid (for example (1) Japanese Laid-Open Patent No. 61-270396, (2) Japanese Laid-Open Patent No. 9-184096 and (3) Japanese Laid-Open Patent No. 2000-323152), and those in which a neutral or weakly acidic solution is used as a process liquid ((4) Japanese Laid-Open Patent Publication No. 10-280163) are known.
In the above-mentioned publication (1), a passivation treatment method for passivation-treating a stainless steel member with for example dilute nitric acid is set forth.
In the above-mentioned publication (2), a surface treatment method for electrolytically treating stainless steel with an aqueous solution of nitric acid and chromic acid is set forth.
In the above-mentioned publication (3), a separator manufacturing method is set forth in which stainless steel is acid-washed with a mixture of nitric acid and hydrofluoric acid and then a passive film is formed with nitric acid.
In the above-mentioned publication (4), a passivation treatment method is set forth in which a stainless steel sheet is coated with a solution in liquid film form containing a neutral salt electrolyte and hydrogen peroxide.
In the above-mentioned publication (1), publication (2) and publication (3), in each case, because an acid is used for the passivation treatment, metal ions solve out into the process liquid. For example, in the case of nitric acid, Ni2+ and Cr6+ solve out. Consequently, a great deal of cost is entailed in processing waste liquid including metal ions after the passivation treatment is finished.
And in the above-mentioned publication (4), because the process liquid is applied in a liquid film state, that is, applied for example using a brush or the like, by spraying using an atomizer, or by repeated dipping and exposure to the atmosphere, the number of steps is large, productivity is low, and as a result cost is high.
Also, as stainless steel treatment methods of this kind, those in which degreasing and passivation treatment are carried out (for example (5) Japanese Laid-Open Patent Publication No. 10-503241), those in which degreasing and etching are carried out (for example (6) Japanese Laid-Open Patent Publication No. 9-291400), those in which degreasing and polishing are carried out (for example (7) Japanese Laid-Open Patent Publication No. 2000-282276), and those in which acid cleaning are carried out (for example (8) Japanese Laid-Open Patent Publication No. 2001-214286) are known.
The above-mentioned publication (5) provides an alkali-based composition for cleaning and passivating the surface of a stainless steel sheet, and a stainless steel surface cleaning liquid is made up of an alkaline component other than alkaline salts such as carbonates+a chelating agent+water+a surfactant.
In the above-mentioned publication (6), electrolysis treatment is carried out with an aqueous solution of pH 10 to 12.5 including sodium hydroxide and sulfuric acid as an electrolyte and thereby rolling oil attached to the surface after cold-rolling of the stainless steel is removed and the concentration of Cr component in the passive film formed on the stainless steel surface is made low and the etchability of the material is improved.
In the above-mentioned publication (7), an alkaline solution is blown at the surface of a cold-rolled stainless steel sheet while it is brushed, whereby smudges (dirt) existing on the surface of the steel sheet after the cold-rolling are removed, and then polishing is carried out, whereby a stainless steel polished product having a surface nature with no pattern or pit flaws is manufactured.
In the above-mentioned publication (8), the stainless steel is acid-washed with an acidic aqueous solution to expose at least one of carbide metal inclusions and boride metal inclusions having conductivity at its surface, and then a neutralizing treatment is carried out by means of an alkaline solution of pH 7 or greater to prevent any increase in electrical contact resistance, and then rinsing with water and drying are further carried out.
The acid cleaning is carried out by immersing the stainless steel in the acidic aqueous solution or showering the acidic aqueous solution onto the stainless steel surface.
Because in the above-mentioned publication (5) the stainless steel sheet is immersed in an alkali-based solution and in the above-mentioned publication (6) the cold-rolled stainless steel is immersed in an aqueous solution including sodium hydroxide and sulfuric acid, for example if cleaning is carried out by an overflow immersion method in which the process liquid is made to overflow from the process tank during immersion, the amount of process liquid used becomes large, and, because with the immersion method the cleaning is carried out gradually, the process time becomes long as well. Also, if the amount of grease attached to the stainless steel material is large, grease may remain there after the cleaning, and certain cleaning sometimes cannot be carried out.
In the publication (8), because different acid and alkali solutions are used, processing of waste liquids must be carried out separately, and costs mount up.
Also, in the publication (7), to effect brushing while an alkaline solution is blown at the stainless steel sheet, a drive source for producing a driving force for brushing is necessary. If processing can be carried out with simple equipment, the cost of the equipment can be reduced.